The present invention relates to a high-voltage power supply circuit with a voltage discharging loop in which the execution of the control program of the microcontroller controls the setting of the frequency of the sine wave signal, the start of the sine wave signal, the end of the sine wave signal, and the opening or closing of the relay switches; in which the sine wave signal from the sine wave generating circuit is amplified by an amplifier, boosted by a transformer, rectified by a rectifier diode, and then filtered by a filter capacitor for output, or for feeding back to a negative terminal of the amplifier for comparison with the voltage at its positive terminal so as to stabilize the output voltage; and in which the execution of the control program of the microcontroller can also control the filter capacitor to discharge the voltage and the transformer to prevent damage to the switches.
FIG. 3 shows a DC high-voltage generating circuit according to the prior art. As illustrated, the input terminal of the self-coupled transformer T1 is connected to an AC power supply. The output terminal of the transformer T1 is connected to the primary side of the transformer T2 through a switch SW1. The secondary side of the transformer T2 is connected to the positive terminal of a rectifier diode D. The negative terminal of the rectifier diode D is connected to the positive terminal thereof by a switch SW3 and also connected to a filter capacitor C through another switch SW4. The output terminal of the DC high-voltage generating circuit is connected to a switch SW2. When the switches SW1 and SW4 are turned on, the transformer T2 is driven to boost the voltage from the self-coupled transformer T1, and to send the boosted voltage to the rectifier diode D for rectification and then to the filter capacitor C for filtration, and therefore a DC high-voltage is obtained from the filter capacitor C. The working frequency of this circuit can only be maintained within 50-60 Hz. The capacity of the filter capacitor C must be big enough to reduce ripple voltage. When the switches SW2 and SW4 are maintained on, the switches SW1 and SW3 are maintained off, and the filter capacitor C forms, with the switch SW2, a discharging loop to discharge the voltage from the filter capacitor C. However, because the switch SW2 bears the voltage from the filter capacitor C, the contacts of the switch SW2 tend to be damaged by the big volume of electric current passing through. Furthermore, this power supply circuit has no voltage feedback means for stabilizing and the voltage, the output voltage of the power supply circuit thus tends to vary with its input voltage.